International Association
for Cryptologic Research

Outsourcing paradigm is one of the most attractive benefits of cloud computing, where computation workloads can be outsourced to cloud servers by the resource-constrained devices, such as RFID tags. With this paradigm, cloud users can avoid setting up their own infrastructures. As a result, some new challenges, such as security and checkability, are inevitably introduced. In this paper, we address the problem of secure outsourcing algorithm for modular exponentiations in the one-malicious version of two untrusted program model. We show that our proposed algorithm is more efficient than the state-of-the-art algorithms. On the other hand, we point out in this paper that the first outsource-secure algorithm for simultaneous modular exponentiations proposed recently is insecure, where the sensitive information can be leaked to the malicious servers. As a result, we propose a new and more efficient algorithm for simultaneous modular exponentiations. We also propose the constructions for outsource-secure Cramer-Shoup encryptions and Schnorr signatures which are also more efficient than the state-of-the-art algorithms.